In most countries, the frequency spectrum is divided between different activities or actors. For example, part of the available frequencies may be reserved for a certain radio access technology and/or for a certain operator of a wireless communication system.
The amount of frequency resources is limited, and the demand for bandwidth is high, making reserved frequency resources very expensive. In order to use the frequency resources more efficiently, the idea of sharing unlicensed bandwidth has been introduced. In line with this, a new concept has been developed: the LTE-U, i.e. LTE in Unlicensed. According to the LTE-U concept, an LTE system can be applied to other frequencies than the ones standardised for LTE in 3rd Generation Partnership Project, 3GPP, standards.
The first implementation of the LTE-U most probably will take place in USA, and the focus in regard of unlicensed frequency bands has particularly been directed to the 5 GHz band, which is currently used mostly for Wifi. It has been proposed to first deploy LTE-U as a Supplemental Downlink, SDL, in the frequency band 5725-5850 MHz in USA, together with a Primary Cell, PCell, operating on a carrier in a licensed band. As an example 3GPP specified band 4 may be used for the Primary Cell.
An air interface for LTE-U is shown in FIG. 1. The wireless device, WD, 101 is connected to an LTE network via two air-interfaces: the Primary Carrier 102 which is in a 3GPP LTE frequency band, and the Secondary Carrier 103 which is in an unlicensed frequency band, e.g. an ISM band. In the first implementations the Secondary Carrier will operate in an SDL mode, only carrying data in the Downlink, DL direction, i.e. from the Radio Access, RA, node to a WD. The RA node could alternatively be denoted e.g. Radio Base Station, RBS, and the WD could alternatively be denoted e.g. User Equipment, UE.
A Primary Carrier 102, or PCell Carrier, may be configured for LTE FDD or TDD, and is intended to provide control signalling, mobility and user data in a licensed or otherwise reserved frequency band. A Secondary Carrier 103, on the other hand, will be configured in an unlicensed spectrum, e.g. with so-called Best-effort user data in the downlink. In the future, the Secondary Carrier is intended to also carry uplink data from a WD to an RA node. Then, the Secondary Carrier may be used as a bi-directional communication channel, and therefore, a two headed arrow has been used to illustrate this carrier in FIG. 1.
The first LTE-U RA node implementations are assumed to be similar to the one shown in FIG. 2. In FIG. 2, the LTE RA node, or eNB, is a dual band RA node, where the first operational band, i.e. the Primary Cell, is operating in 3GPP band 4 for transmission and reception, and the SDL cell, which is downlink-only operates at 5725-5850 MHz, which is a frequency band that can also be used by e.g. 5 GHz Wifi.
Thus, an RA node is needed, which is operable in dual bands, of which at least one may be unlicensed.